Method of producing press-hardened structural parts

ABSTRACT

In a method, a blank of unhardened steel sheet is subjected in a press tool to a hot forming and press-hardening process to produce a structural part. After press-hardening, at least one linear zone of the structural part is heat treated in order to increase ductility and reduce strength in the linear zone in relation to adjacent regions of the structural part. After heat treatment, a bending or cutting operation is carried out on the structural part along the linear zone.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the priorities of German Patent Application, Serial No. 10 2010 011 368.9-14, filed Mar. 12, 2010, and European Patent Application, Serial No. 10 008 035.7, filed Aug. 2, 2010, pursuant to 35 U.S.C. 119(a)-(d), the contents of which are incorporated herein by reference in its entirety as if fully set forth herein.

BACKGROUND OF THE INVENTION

The present invention relates to a method of producing press-hardened structural parts, in particular for a body or components of motor vehicles.

The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.

To decrease the total weight and to reduce fuel consumption of motor vehicles, the so called New Steel Body concept has been developed in recent years, involving lightweight high-strength steel construction that allows designers to save weight by reducing the sheet metal thickness and thus the total weight of the vehicle. Such vehicle components of high strength steel normally undergo heat treatment and press-hardening.

U.S. Pat. No. 5,972,134 discloses a process for the manufacture of a metallic molded structural part for motor vehicle components such as door impact girders or bumpers with areas with a higher ductility in relation to the rest of the structural component part. For this purpose, partial areas of a plate are initially heated to a temperature of 900° C. within a period of less than 30 seconds. Subsequently, the thermal-treated plate is shaped in a pressing tool to form the molded structural part and is heat-treated in the pressing tool. This conventional process suffers shortcomings in particular, when narrow and especially linear areas are subjected to heat treatment. Temperatures reaching up to 900° C. cause warping and deformation of the structural part and may cause the heat-treated area of the structural part to harden again because the steel microstructure depending on the composition is not only tempered starting from a temperature of 723° C. but is also partly austenitized. Further, this conventional process renders a non-warping securement of the structural part difficult because of the use of a conveyor device.

Materials used during heat treatment and press-hardening and the resultant structural part have high strengths of 700 MPa and more during shaping. At such strengths, the risk of a delayed cracking and the possibility of unpredictable failure of the structural parts rises in view of the presence of hydrogen which can be absorbed during production (metallurgical hydrogen) or as a result of surface treatment or when the material corrodes.

A delayed cracking may also be encountered after undergoing a plastic deformation during which the surface is activated. (Hard) cut edges of high or super high strength structures that are exposed to a corrosive environment in the presence of atomic hydrogen tend to brittle in the area of the cut edges as a result of inherent stress and thus tend to encounter a delayed cracking. When heat treated and press-hardened motor vehicle components such as B pillars for example are involved, such a hydrogen embrittlement of trimmed edges may lead to reduced energy absorption in the event of a crash.

It would therefore be desirable and advantageous to address these problems and to obviate other prior art shortcomings.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a method includes the steps of subjecting a blank of unhardened steel sheet in a press tool to a hot forming and press-hardening process to produce a structural part, heat treating at least one linear zone of the structural part, and subjecting the structural part after the heat treating step to a bending or cutting operation along the linear zone.

In accordance with the present invention, the mechanical properties of a press-hardened structural part is optimized in a narrow linear zone through heat treatment in order to permit subsequent bending or cutting operations to be executed simpler and more cost-efficient. The structural part that has been completely through hardened is hereby heat treated along a linear zone commensurate with a later bending line or trimming line after being hot formed. As a result, strength is reduced in the linear zone and ductility is increased. The hot formed and press-hardened structural part can then in an optimum manner undergo a bending or cutting operation along the linear zone. The processing steps are thus efficient, cost saving, and used processing machines are subject to less wear. In addition, the method according to the present invention results in a resistance to a delayed cracking of the structural parts.

Critical parameters that promote delayed cracking include i.a. local accumulation of hydrogen atoms and the presence of residual or internal stress. Thus, the present invention proposes to reduce the strength of edges of the structural part, especially the cut edges, and to increase ductility. By targeting the heat treatment, e.g. inductive heat treatment, along the trimming line after the hot forming process, mechanical properties of the fully through hardened structural part are locally optimized along the cut edges and/or in the regions adjacent to the cut edges so that the tendency to embrittlement and cracking are minimized or even eliminated.

After hot forming and press-hardening, the structural part has a tensile strength of at least 700 MPa. Currently preferred is a tensile strength of 1,000 MPa to 1,500 MPa.

According to another advantageous feature of the present invention, the ductility in the linear zone can be increased by at least 5% in relation to a region of the structural part that is adjacent to the linear zone and does not undergo a heat treatment.

The hot formed and press-hardened structural part is suitably clamped and fixed in position during heat treatment in a holding device. The heat treatment can hereby be carried out within the holding device in accordance with a temperature-time diagram that is specific for the structural part at hand. The linear zone of the structural part can be subjected to heat treatment at a temperature between 100° C. and 700° C. over a time period of less than or equal to (≦)30 seconds. As a result, ductility is locally increased in the linear zone and the strength reduced.

According to another advantageous feature of the present invention, the heat treating step can be executed inductively. As an alternative, the heat treating step may also be executed through infrared radiation. Inductive heat treatment is applicable for very narrow linear zones of the structural part. A very small and fine transition zone can hereby be created between regions of high strength and high hardness and regions of less strength and less hardness.

According to another advantageous feature of the present invention, the structural part can be clamped in the holding device until the structural part has cooled down to a temperature level that does not cause distortion. As a result, the structural part does not encounter or encounters only insignificant distortion which is within permissible dimensional tolerances after the heat treatment. The holding device with integrated heating unit, for example integrated inductor, thus forms an important aspect of the present invention.

According to another advantageous feature of the present invention, at least some areas of the structural part can be maintained in the holding device under elastic tension while the linear zone undergoes heat treatment. As a result, the structural part can be further shaped with a desired geometry. In this way, by superimposing forced bending, tensile and pressure forces and heat stress, the properties of the structural part can be tailored in the region undergoing heat treatment.

According to another advantageous feature of the present invention, the structural part can be secured in the holding device in such a way that geometrical changes of the structural part are compensated during the heat treating step so that the structural part has a desired geometry after undergoing heat treatment and/or removal from the holding device. There is thus no need to necessarily correspond the contour of the holding device to the desired end geometry of the structural part. Geometric changes of the structural part are taken into account during heat treatment and utilized during configuration of the structural part and production of the linear zone that has changed material microstructure.

As described above, the hardened structural part is heated in the holding device for a time period of 30 seconds to a temperature between 100° C. and 700° C. Suitably, the heat treatment is executed over a time period of less than 10 seconds. Currently preferred is a time period of 2 seconds. As the temperature for heat treatment is below 700° C., re-hardening of the heat treated zone is avoided.

According to another advantageous feature of the present invention, the structural part may be provided with a surface coating. The surface coating may be applied onto the blank before undergoing heat treatment and press-hardening or after subjecting the linear zone to heat treatment. Examples of a pre-coated blank as starting material include hot dip aluminized blank, a blank coated with an aluminum-silicon alloy, or a blank coated with a zinc or aluminum-zinc alloy. A steel blank used as starting material is thus provided with a protection against corrosion already before hot forming and press-hardening. The protection against corrosion may be based on a light metal alloy.

BRIEF DESCRIPTION OF THE DRAWING

Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:

FIG. 1 is a perspective illustration of an inductor for use in the production of a structural part in accordance with to the present invention;

FIG. 2 is a perspective illustration of a holding device with integrated inductor for use in the production of a structural part in accordance with to the present invention;

FIG. 3 is an enlarged detailed view of an area encircled in FIG. 2 and marked “A”;

FIG. 4 is a perspective illustration of a detail of the holding device, depicting the disposition of the inductor; and

FIG. 5 a is a schematic illustration of a detail of a structural part;

FIG. 5 b is a schematic illustration of a side area of the structural part with bent longitudinal edge; and

FIG. 5 c is a schematic illustration of the side area of the structural part with trimmed longitudinal edge.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Throughout all the figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.

Turning now to the drawing, and in particular to FIG. 1, there is shown a perspective illustration of an inductor, generally designated by reference numeral 1, for use in the production of a structural part 5 (FIGS. 5 a, 5 b) in accordance with the present invention. Although the use of an inductor is described here by way of example, it is of course also conceivable to use infrared radiation for example as an alternative.

The inductor 1 includes a narrow square copper tube 2 which has three sides facing away from the surface of the structural part being treated and surrounded by concentrator sheets 3, 4, respectively. In practical application, the copper tube 2 may have a width of 4 millimeters and a height of 8 millimeters. The concentrator sheets 3, 4 extend over the entire length of the copper tube 2 and have a thickness of 4 millimeters. The inductor 1 has a longitudinal dimension which is a multiple of the width thereof, and may be configured with several curves in the course of its longitudinal dimension. The geometric contour and curved profile is suited to the heat treatment being carried out on the structural part 5, as will be described in greater detail hereinafter,

FIGS. 2 and 3 show the inductor 1 integrated in a holding device 6. The holding device 6 essentially includes a top tool 7 and a bottom tool 8. Both, top and bottom tools 7, 8 are made of light metal, such as aluminum or aluminum alloy. Aluminum or aluminum alloy have the benefit that the material is heat conducting.

Production of a structural part 5 of complex configuration, for use in a body or as component of a motor vehicle, is carried out as follows. A blank of non-hardened and hot formable steel sheet is heated to a temperature above the austenitizing temperature and hot formed in a press tool into the structural part 5 which is clamped in the press tool and undergoes press-hardening. The structural part 5 has a tensile strength of at least 700 MPa. Currently preferred is a structural part 5 after undergoing hot forming and press-hardening with a tensile strength in a range between 1,000 MPa and 1,500 MPa. The thus press-hardened structural part 5 is removed from the press tool and a targeted narrow linear zone B of the structural part 5 is then subjected to a heat treatment. Although the disclosure relates to the presence of one linear zone B, it will be understood by persons skilled in the art that this is done by way of example only, as the structural part may, of course, have several linear zones that are subjected locally to heat treatment in order to reduce strength and increase ductility.

The linear zone B corresponds hereby to the course of a later bending line or trimming line of the structural part 5, as will be described hereinafter with reference to FIGS. 5 a and 5 b.

The heat treatment takes place in the holding device 6 in which the hot formed and press-hardened structural part 5 is either received and clamped entirely in a form-fitting manner or by linear support surfaces 9, 10, shown in FIG. 3. The support surfaces 9, 10 extend along the structural part contour on both sides in correspondence to the course of the linear zone B and inductor 1, i.e. the bending or trimming line. In the illustrated example, the inductor 1 is integrated in an oblong hole 11 of the holding device 6, as shown in FIG. 4. The inductor 1 is suitably integrated in the upper tool 7, although it is, of course, conceivable to integrate the inductor in the lower tool 8. The inductor 1 with its square copper tube 2 and the surrounding concentrator sheets 3, 4 is arranged in the holding device 6 such as to have a same distance to the surface of the structural part 5 over the entire length. Currently preferred is a distance of 2 millimeters.

As the structural part 5 is securely clamped in the holding device 6, the linear zone B is subjected to a heat treatment. The heat treatment is performed inductively by means of the inductor 1. The linear zone B of the hardened structural part 5 is hereby heated to a temperature between 100° C. and 700° C. for a time period of less than or equal to 30 seconds. Currently preferred is a time period of 2 seconds. The heat treatment of the linear zone B increases its ductility and reduces its strength. The holding device 6 retains the shape of the inductively heated structural part 5, normally maximal 30 seconds, until the temperature is lowered to a level that does not cause warping or distortion of the structural part 5.

The heat treatment and the resultant change in material properties can be carried out in a very precise manner and tailored to a very small or narrow but relative long linear region (zone B) with fine transitions. The tailored heat treatment prevents the drawbacks of hydrogen embrittlement, in particular hydrogen-induced cracking. A localized accumulation of hydrogen atoms and the presence of residual or internal tensions are reduced by the heat treatment to uncritical levels.

FIGS. 5 a, 5 b, 5 c show schematically a hot formed and press-hardened structural part 5. FIG. 5 a shows the three-dimensional structural part 5 after the linear zone B underwent heat treatment to alter its material property. The targeted heat treatment at a temperature between 100° C. and 700° C. for a time period of 30 seconds increases ductility and reduces strength.

Following the heat treatment in the holding device 6, a bending operation and/or trimming operation is carried out on the structural part 5 along the linear zone B. FIG. 5 b shows a right-hand side area 12 of the structural part 5 with bent longitudinal edge 13. The ductile strength-reducing linear zone B extends at the outer free end 14 of the bent side area 12 along the curved region of the longitudinal edge over the length of the structural part 5.

FIG. 5 c shows the side area 12 of the structural part 5 with trimmed edge 15. The ductile strength-reducing linear zone B extends at the outer trimmed edge 15 over the length of the structural part 5. The cutting operation has been executed within the linear zone B. The side area 12 is further bent in the linear zone B.

Although not shown in detail, the structural part 5 can be provided with a surface coating. Application of the surface coating may be carried out after heat treatment and following bending or trimming operations or also prior thereto.

It is also conceivable to use a pre-coated blank as starting material for producing a press-hardened structural part 5. Heat treatment of a linear zone B can then be carried out, as described above, on the press-hardened structural part produced from the pre-coated blank.

It is further possible to maintain at least some areas of the structural part 5 in the holding device 6 under elastic tension during heat treatment. The structural part 5 is hereby fixed in the holding device 6 in a way as to take into account geometrical changes of the structural part 5 as a result of the heat treatment so that the structural part 5 has the desired geometry after heat treatment and/or removal from the holding device 6.

While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit and scope of the present invention. The embodiments were chosen and described in order to explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein: 

What is claimed is:
 1. A method, comprising the steps of: subjecting a blank of unhardened steel sheet in a press tool to a hot forming and press-hardening process to produce a structural part; heat treating at least one linear zone of the structural part; and subjecting the structural part after the heat treating step to a bending or cutting operation along the linear zone.
 2. The method of claim 1, wherein the structural part has a tensile strength of at least 700 MPa after undergoing press-hardening.
 3. The method of claim 1, wherein the structural part has a ductility which is increased in the linear zone in relation to a region adjacent to the linear zone.
 4. The method of claim 3, wherein the ductility in the linear zone is increased by at least 5% in relation to the adjacent region of the structural part.
 5. The method of claim 1, wherein the heat treating step is executed at a temperature between 100° C. and 700° C. over a time period of less than or equal to 30 seconds.
 6. The method of claim 1, wherein the heat treating step is executed inductively.
 7. The method of claim 1, wherein the heat treating step is executed through infrared radiation.
 8. The method of claim 1, wherein the heat treating step is executed while the structural part is clamped in a holding device.
 9. The method of claim 8, wherein the structural part is clamped in the holding device until the structural part has cooled down to a temperature level that does not cause distortion.
 10. The method of claim 8, wherein at least one area of the structural part is maintained in the holding device under elastic tension.
 11. The method of claim 8, wherein the structural part is secured in the holding device in such a way as geometrical changes of the structural part are compensated during the heat treating step so that the structural part has a desired geometry after undergoing heat treatment and/or removal from the holding device.
 12. The method of claim 1, further comprising the step of coating a surface of the structural part.
 13. The method of claim 1, further comprising the step of coating the blank before undergoing heat treatment and press-hardening.
 14. The method of claim 1, wherein the structural part is used for a body or component of a motor vehicle. 